Nested loop execution in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested loop in PHP?

A nested loop is a loop inside another loop. The inner loop runs completely every time the outer loop runs once.

beginner

How many times will the inner loop run in this code?

<?php
for ($i = 1; $i <= 3; $i++) {
  for ($j = 1; $j <= 2; $j++) {
    echo "$i,$j ";
  }
}

The inner loop runs 2 times for each of the 3 outer loop runs, so 3 × 2 = 6 times total.

beginner

What is the output of this nested loop?

<?php
for ($i = 1; $i <= 2; $i++) {
  for ($j = 1; $j <= 3; $j++) {
    echo $i * $j . ' ';
  }
}

The output is: 1 2 3 2 4 6
Explanation: For i=1, j=1 to 3 outputs 1,2,3. For i=2, j=1 to 3 outputs 2,4,6.

intermediate

Why do nested loops often have higher time complexity?

Because the inner loop runs completely for each iteration of the outer loop, the total steps multiply, making the program slower as input grows.

beginner

How can you visualize nested loops in real life?

Imagine you have 3 boxes, and inside each box are 2 balls. You open each box (outer loop) and then pick each ball inside (inner loop).

In PHP, if the outer loop runs 4 times and the inner loop runs 5 times, how many times does the inner loop execute in total?

A 20

B 9

C 5

D 4

The inner loop runs 5 times for each of the 4 outer loop runs, so 4 × 5 = 20 times.

What will this PHP code output?

<?php
for ($i=1; $i<=2; $i++) {
  for ($j=1; $j<=2; $j++) {
    echo $i . $j . ' ';
  }
}

A 1 2 2 2

B 11 12 21 22

C 12 21 22 11

D Error

The output is '11 12 21 22 ' because for i=1, j=1 and 2 prints 11 and 12; for i=2, j=1 and 2 prints 21 and 22.

Which statement is true about nested loops?

A Outer loop runs inside inner loop

B Inner loop runs once for all outer loop iterations

C Inner loop runs completely for each outer loop iteration

D Nested loops cannot be used in PHP

The inner loop runs fully every time the outer loop runs once.

What is the time complexity of two nested loops each running n times?

A O(log n)

B O(n)

C O(1)

D O(n^2)

Two nested loops each running n times result in O(n × n) = O(n^2) time complexity.

In PHP, which keyword is used to start a loop?

A for

B loop

C repeat

D iterate

The 'for' keyword is used to start a for loop in PHP.

Explain how nested loops work in PHP with an example.

Think about a loop inside another loop and how many times the inner loop runs.

Describe a real-life situation that helps you understand nested loops.

Think about opening boxes and picking items inside.

Practice

1. What happens when a nested loop runs in PHP?
for ($i = 0; $i < 2; $i++) {
for ($j = 0; $j < 3; $j++) {
// code
}
}

easy

A. The inner loop runs completely for each iteration of the outer loop.

B. The outer loop runs completely for each iteration of the inner loop.

C. Both loops run only once.

D. The loops run simultaneously without nesting.

Solution

1. Step 1: Understand loop nesting order

   The outer loop runs from 0 to 1 (2 times). For each outer loop iteration, the inner loop runs fully.

2. Step 2: Inner loop completes all cycles per outer iteration

   For each $i, $j runs from 0 to 2 (3 times), completing all inner cycles before next outer iteration.

3. Final Answer:

   The inner loop runs completely for each iteration of the outer loop. -> Option A

4. Quick Check:

   Inner loop finishes fully inside outer loop [OK]

Hint: Inner loop runs fully inside each outer loop cycle [OK]

Common Mistakes:

• Thinking outer loop runs inside inner loop
• Assuming loops run only once
• Confusing simultaneous execution

2. Which of the following is the correct syntax for a nested for loop in PHP?

easy

A. for ($i = 0; $i < 2; $i++) { for $j = 0; $j < 3; $j++ { echo $i . $j; } }

B. for ($i = 0; $i < 2; $i++) { for ($j = 0; $j < 3; $j++) { echo $i . $j; } }

C. for ($i = 0; $i < 2; $i++) for ($j = 0; $j < 3; $j++) echo $i . $j;

D. for $i = 0; $i < 2; $i++ { for $j = 0; $j < 3; $j++ { echo $i . $j; } }

Solution

1. Step 1: Check PHP for loop syntax

   PHP requires parentheses around loop conditions and braces for blocks.

2. Step 2: Validate nested loop syntax

   for ($i = 0; $i < 2; $i++) { for ($j = 0; $j < 3; $j++) { echo $i . $j; } } uses correct parentheses and braces for both loops and echo statement inside.

3. Final Answer:

   for ($i = 0; $i < 2; $i++) { for ($j = 0; $j < 3; $j++) { echo $i . $j; } } -> Option B

4. Quick Check:

   Correct parentheses and braces in nested loops [OK]

Hint: Use parentheses and braces properly in PHP loops [OK]

Common Mistakes:

• Omitting parentheses around loop conditions
• Missing braces for loop blocks
• Incorrect for loop syntax without $ sign

3. What is the output of this PHP code?

for ($i = 1; $i <= 2; $i++) {
  for ($j = 1; $j <= 3; $j++) {
    echo $i * $j . ' ';
  }
}

medium

A. 1 2 3 1 2 3

B. 1 4 9 2 5 8

C. 1 2 3 2 4 6

D. 6 12 18 6 12 18

Solution

1. Step 1: Trace outer loop values

   $i runs from 1 to 2.

2. Step 2: Trace inner loop and calculate products

   For $i=1: $j=1,2,3 -> 1*1=1, 1*2=2, 1*3=3
   For $i=2: $j=1,2,3 -> 2*1=2, 2*2=4, 2*3=6

3. Final Answer:

   1 2 3 2 4 6 -> Option C

4. Quick Check:

   Multiply outer and inner loop counters [OK]

Hint: Multiply outer and inner counters for output [OK]

Common Mistakes:

• Mixing up loop variable values
• Printing inner loop values only
• Ignoring multiplication in echo

4. Find the error in this nested loop PHP code:

for ($i = 0; $i < 3; $i++) {
  for ($j = 0; $j < 2; $i++) {
    echo $i . $j . ' ';
  }
}

medium

A. Inner loop increments $i instead of $j causing infinite loop.

B. Outer loop condition is incorrect.

C. Missing semicolon after echo statement.

D. Variables $i and $j are not declared.

Solution

1. Step 1: Check inner loop increment variable

   The inner loop increments $i instead of $j, which is wrong.

2. Step 2: Understand effect of wrong increment

   Incrementing $i inside inner loop changes outer loop variable unexpectedly, causing infinite or wrong behavior.

3. Final Answer:

   Inner loop increments $i instead of $j causing infinite loop. -> Option A

4. Quick Check:

   Increment correct loop variable to avoid errors [OK]

Hint: Increment inner loop variable inside inner loop [OK]

Common Mistakes:

• Incrementing wrong variable in inner loop
• Confusing loop conditions
• Assuming variables need declaration in PHP

5. You want to print a multiplication table from 1 to 3 using nested loops in PHP. Which code correctly prints the table with each row on a new line?

?

hard

A. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j; } echo "\n"; }

B. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . "\n"; } }

C. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } }

D. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } echo "\n"; }

Solution

1. Step 1: Understand output format

   Each row should print products separated by spaces, then a newline after inner loop ends.

2. Step 2: Check each option for correct newline placement

   for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } echo "\n"; } prints products with spaces inside inner loop, then newline after inner loop completes, correctly formatting rows.

3. Final Answer:

   for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } echo "\n"; } -> Option D

4. Quick Check:

   Newline after inner loop completes for each row [OK]

Hint: Print newline after inner loop to separate rows [OK]

Common Mistakes:

• Printing newline inside inner loop
• Missing spaces between numbers
• No newline causing all output in one line